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Summary form only given.VCSELs usually emit linearly polarized light oriented along a crystal axis. However, it is commonly observed that, as the current is further increased, the polarization orientation of the optical field switches to the orthogonal one while preserving the emission profile. This polarization switching (PS) has been explained as a result of thermal heating. This mechanism, however, cannot explain a recent experiment where PS was observed while keeping constant the VCSEL temperature. This experimental observation can also not be explained just by the linear effect of cavity anisotropies, a mechanism that also influences PS. An alternative explanation is offered by the spin-flip model (SFM), which takes into account the spin dynamics of carriers. Carrier's spin influence on the lasing process in VCSELs can be enhanced by considering a VCSEL optically pumped with elliptically polarized light in a transverse magnetic field. In this case, the combination of optical pump with Larmor precession, birefringence and spin relaxation leads to polarization oscillations in the VCSEL emission in continuous as well as in pulsed optical pump. The SFM model has been extended in order to incorporate all those effects.